1. Field of the Invention
This invention relates to composite members of the type wherein two or more molded members are strongly integrated and more particularly, to polyfunctional composite members wherein when applied as electronic, electric or mechanical parts, they are kept stably integrated over a long term especially at bonded portions of the two or more members. The invention also relates to method for making the composite members of the type mentioned above. Only for convenience's sake, the composite members to which the invention is directed are described herein as applied as a rotor in spindle motors for hard disk drives.
2. Description of the Prior Art
A rotor in spindle motors for hard disk drives has been constituted of a composite member which is made of an iron member of a circular form integrated with an aluminum member of an inverted pot shape. The iron member shows the function as a magnetic circuit and the aluminum member takes part in a corrosion resistance and precise cuttability.
FIG. 5 shows an illustrative view, in section, of the most popular method of fabricating the rotor. In the figure, a separately provided iron pipe member 2 is placed in a cavity 3 of molds 1a, 1b as shown, and an aluminum melt is introduced into the cavity 3 through a melt passage 1c to integrally mold with the pipe 2. Subsequently, the molding product is precisely finished by machining and cutting.
Aside from the above fabrication method, there is known an integrating method wherein an iron member and an aluminum member are separately formed and subjected to shrinkage fit, expansion fit or mechanical caulking fits such as serration and knurling. The members may be not only cast and mechanical processed products, but also powder-sintered products.
However, the iron member and the aluminum member integrally formed by the casting are combined together only by restriction and mechanical caulking fit caused by shrinkage through solidification of the aluminum melt. In this condition, when an external force is applied to by machining during a fabrication process or when a heat cycle having a great temperature difference in a working environment is repeatedly suffered such as in the case of the rotor, there is indicated the problem that both members are liable to be loosened owing to the difference in thermal expansion therebetween.
In the casting procedure of aluminum, cavities may be formed in the aluminum member, or impurities or inclusions may be involved in the member. For utilization as the rotor of a spindle motor which is required to have a high precision that an error in revolution should be not larger than 0.25 .mu.m, there has been a problem with respect to the precision.
As stated before, the use of the powder-sintered product instead of the cast product has been considered. In this connection, however, for the formation of integrally sintered molding products through HIP treatment of different kinds of metal powders, a plurlaity of powdery materials have to be properly formulated at desired positions. This is not technically completed from the standpoint of production techniques. In addition, this method has a great problem on cost.
We developed composite materials such as disclosed, for example, in U.S Pat. No. 4,943,319, wherein ferromagnetic powders such as of Fe and metallic powders such as Al are admixed and pressed for diffusion bonding to obtain composite materials. These composite materials have been provided as a polyfunctional material of a novel type which has not only light weight, good workability and electricl conductivity, but also ferromagnetic characteristics of Fe, and have now been put into practice as an electromagnetic shield material or a rotor material for induction motor. These materials are in a form where a function-imparting powder is uniformly dispersed in a matrix powder. Accordingly, this type of composite material cannot be applied to fields of parts, such as a rotor for spindle motors as shown in FIGS. 3 and 4, wherein portions 4, 5, 7 and 9 of a part are required to have different functions and are completely integrated into the part. Although the respective portions may be formed while taking ingredients and amounts of powders to be combined into account, a technique of integrating these portions in comformity with required strength has not been developed yet as stated before.